The completely dominating method for producing ammonia is thus based on the equilibrium equation EQU N.sub.2 +3H.sub.2 .revreaction.2NH.sub.3 +heat.
Since heat is generated when ammonia is formed by this reaction, the concentration or yield of ammonia at equilibrium is decreased when the temperature increases. The reaction is, however, very slow at moderate temperatures, and in industrial applications it has been necessary to carry out the reaction at increased temperatures, thereby accepting a relatively low yield per unit operation in order to attain an economical production rate.
The total number of gas molecules present decreases when ammonia is formed, which means that the yield increases when the pressure is increased. The reaction rate also increases with increased pressures. For these reasons, a pressure of between 120 and 600 atmospheres has been used in different industrial forms of the basic process, e.g. Mont-Cenis, Haber-Bosch and Casale. The use of high pressures makes it necessary to use massive pressure vessels and effective gas pumps, which naturally make capital costs extremely high.
Direct synthesis of ammonia on a large scale did not become economically possible until Haber discovered a catalyst which noticeably increased the reaction rate, even at comparatively moderate temperatures. This made it possible to achieve a practical yield of about 30% ammonia at temperatures of between 350.degree. C. and 600.degree. C.
The catalyst used at present in the Haber-Bosch process consists of a mixture of iron oxide and relatively small amounts of oxides of lighter metals, e.g. potassium and aluminium. Modern plants in which this type of catalyst are used can be operated at 350.degree.-400.degree. C. and 200-350 atmospheres and give a yield of about 15-20% per unit operation.
Since the manufacture of ammonia is to a large extent dependent on the availability of cheap hydrogen gas, large efforts have been made to find methods for cheapening the production of hydrogen and for modifying the Haber-Bosch process after manufacturing the hydrogen gas. The transfer from coal to naphta as raw material for producing hydrogen gas meant, for example, that the manufacturing costs for ammonia could be halved during the 1960's. However, further radical improvements in this area are hardly to be expected, due to the continually increasing costs of raw materials.
Parallel to the work of obtaining cheaper hydrogen gas, research has been directed towards new catalysts, ever since Haber discovered his catalysts, for use in the direct synthesis of ammonia via hydrogen and nitrogen. Innumerable materials and combinations of materials have been tested, and certain improvements have been reported, but it is evident that no revolutionary discovery has been made, since the oxides mentioned above are the catalysts still in practical use.
Since innumerable catalysts have been tested during the years, it would appear to be almost impossible to summarize all the experiments that have been made, and even less possible to draw any conclusions as to how the catalyst could be improved. The majority are, however, of the Haber-type, i.e. they consist of heterogenous mixtures of metals and metal oxides. The catalysts, thus, consist of a mixture of many phases, including metals, and are usually produced by fusion at high temperatures and reduction. The first patent concerning such mixtures for catalysts in the synthesis of ammonia was granted to F. Haber about 70 years ago. Since then a number of catalysts of this type have been patented, in these the metals in the Haber mixture have completely or partially been replaced by other similar metals. Thus, e.g. chromium, cobalt or nickel have been used instead of iron, with sodium or magnesium as the light metal. As examples of heterogenous Haber-type mixtures can be mentioned those mixtures of vanadium and nickel and of magnesium and nickel, respectively, which are disclosed in the Swedish Pat. No. 131,225. It is true that it is implied in said patent specification that certain improvements with regard to pressure and temperature should be obtainable with the catalyst described, but obviously no drastic improvements have been obtained. What is still generally applicable to the Haber-type catalysts is therefore, that apart from consisting of heterogenous mixtures, they can not be said to be effective until at a temperature exceeding about 300.degree. C.
It has further been found that intermetallic compounds based on rare earth metals are utilizable as catalysts in the synthesis of ammonia, and this has i.a. been disclosed by W. E. Wallace in a publication with the title: "Rare Earth and Actinide Intermetallics as Hydrogenation Catalysts" published at the International Symposium on Hydrides for Energy Storage in Geilo, Norway, on 14-19 August, 1977. In said publication it is stated that the new catalysts are not better than a known commercial catalyst, calculated on the basis of weight, but that they have greater activity expressed as conversion per m.sup.2 of catalyst. There is nothing to be read concerning noticeable general improvement for the new catalysts, however, especially if it is born in mind that the price of a catalyst based on a rare earth metal ought to be comparatively high.